A charge pump is a basic circuit arrangement that charges a capacitor to an input voltage in a charge phase and discharges the capacitor into a load in a discharge phase. A switching arrangement connects the capacitor in series with the input voltage source so that the output voltage applied to the load is twice the input voltage. With the same concept, plural capacitors can each be charged to the input voltage in the charge phase and connected in series with each other and with the input voltage source in the discharge phase. When the output voltage of the charge pump is twice the input voltage, the charge pump is referred to as a voltage doubler. When the output voltage is three times the input voltage, the charge pump is a DC-DC boost converter comprising a charge pump voltage tripler.
With an appropriate design of the charge pump and of the associated switching arrangement, it is possible to build a DC-DC boost converter capable of operating in either of a voltage doubler or voltage tripler mode. This kind of converter is needed in certain mobile applications where the input voltage source is a battery which has a variable voltage over lifetime or charge condition, and also where the required supply voltage may vary. Typical applications of this kind are LCD or TFT displays. In a converter of this kind, the charge pump operates in the doubler mode as long as the required supply voltage is achieved by doubling the input voltage from the battery. When the required supply voltage exceeds twice the battery voltage, the charge pump is switched to the tripler mode.
While the charge pump is in the tripler mode, the requirements for the output voltage may change. If a new requirement is for an output voltage that can be achieved in the doubler mode, and the discharge pump is in the tripler mode, it is not possible without a complicated load prediction mechanism to switch back to the doubler mode. In that case, however, the converter must regulate the tripled voltage down to the required output voltage, thereby wasting a lot of energy.